Wireless communication has been used as a technique for eliminating load of wiring work in conventional cable communication, and further realizing mobile communications. For example, a wireless LAN (Local Area Network) employing a typical standard such as IEEE (The Institute of Electrical and Electronics Engineers) 802.11 or the like has already come into widespread use. Further, recently, a broadband Internet connection service employing a wireless LAN, of which a “hot spot (registered trademark)” is representative, is coming into common use. Specifically, a base station or access point is installed in a café, hotel, fast-food shop, train station, airport, or the like, and a service area where the Internet is available is provided. A user can receive a service such as IP (Internet Protocol) phone or access to WWW (World Wide Web) information space, or the like even when away from the home or office. Therefore, many information devices serving as a client terminal have often been equipped with a wireless LAN function.
With a wireless communication system made up of a server and client terminals, transfer of information or synchronous processing is mutually performed. Now, many of the client terminals are battery-driven mobile terminals such as cellular phones or PDAs (Personal Digital Assistants) or the like, and accordingly, it is desirable to realize energy saving by shutting down transmitters/receivers when unnecessary.
In the case of a cellular phone, when being in and out of service such as when in a subway, inactivating the communication function when out of service improves the effects of power saving. Known examples include a function for manually turning just the function of a communication system to an off mode, and a function for prolonging the sensing interval as to a base station on a step-by-step basis. However, it is burdensome work for a user to manually perform switching of the operation mode each time the cellular phone experiences being in and out of service, which is poor in practicality.
Also, with a wireless LAN, scan operation for confirming a network existing in the current location is needed. There are two types of scan procedures; active scan and passive scan. With active scan, a client terminal periodically transmits a probe request frame to sense an access point. On the other hand, with passive scan, a client terminal repeats an awake mode for activating a reception state only at the time of reception of a beacon frame from the access point, and a sleep mode for inactivating a reception state without performing transmission by itself. According to passive scan, the transmitter/receiver is suspended under the sleep mode, and accordingly, battery consumption can be suppressed. Also, with a power saving mode at the time of passive scan, the number of times of awake is reduced by thinning out reception of a beacon, whereby power consumption can further be reduced. However, even if operation time is shortened, the reception circuit has to periodically operate for reception of a beacon, and accordingly, the operation of the transmitter/receiver is not completely suspended. Also, a user performs switching of the operation mode of the wireless LAN function manually or using a timer or the like, which is poor in practicality in the same way as described above.
Also, with wireless communication, a situation can be prevented such as loss of synchronous data, partial synchronous processing of data, or the like, by determining whether or not to perform synchronous processing using the value of received field strength (e.g., see PTL 1). However, with synchronous processing determination using received field strength, the received field strength has to be obtained in preparation for the synchronous processing, and the communication function has to constantly operate even in an offline state, and consequently, power consumption increases.
Also, there has been proposed an information distribution system for detecting whether or not wireless communication between an external device and a recording apparatus can be performed according to the signal quality of wireless communication (e.g., see PTL 2). However, in order to sense the signal quality, the communication function needs to constantly operate even in an offline state in the same way as described above, and consequently, power consumption increases.
Also, there has been proposed a network system for performing scheduling of synchronous processing using a method such that synchronous timing is periodically provided using a timer of an information terminal, synchronous processing is started when the processing load of the information terminal reaches equal to or smaller than a stipulated value (so as not to prevent the operation of an application that runs in parallel), or the information terminal monitors the communication load of the network, and when the communication load reaches equal to or smaller than a stipulated value, synchronous processing is started, or synchronous processing is started by a user using a synchronous start operation (e.g., see PTL 3). However, the communication function of the terminal needs to operate for the information terminal monitoring the communication load of the network, and consequently, power consumption increases.